The proposed research is a multicenter prospective study to develop and validate widely applicable criteria for stratifying risk of cardiac death in the year following acute myocardial infarction. A homogeneous population (acute myocardial infarction) derived from a heterogeneous group of hospitals (university and community) widely dispersed in geography (New York, St. Louis, Rochester) will be studied to permit generalization of the results to the whole population at risk. Three special studies have been included in the clinical evaluation of the postinfarction population prior to hospital discharge: radionuclide ejection fractions for quantitative evaluation of left ventricular function, 24-hour Holter ECG tape recordings for the precise quantitation of the degree of electrical instability, and limited activity tests to assess overall cardiovascular function and the presence of activity-related myocardia ischemia. Approximately 1000 patients who survive the early hospital phase of an acute myocardial infarction will be entered during a 24-month enrollment period. Routine clinical evaluation and special studies will be performed prior to hospital discharge. All patients will be followed for a minimum of one year with clinical follow-up evaluation at 3, 6 and 12 months. A standard Bruce-protocol exercise will be obtained at the three month follow-up visit. The main thrust of the data analysis are: 1) to identify functional risk variables which can be widely applied to risk stratification in postinfarction patients; 2) to elucidate pathophysiologic mechanisms responsible for survival and cardiac death by uncovering interactions and associations among the risk indicators; 3) to critically evaluate existing clinical concepts involving postinfarction patients by testing a priori hypotheses on this prospective population; 4) to characterize current patterns of therapy in postinfarction patients to improve background information for the design of future therapeutic intervention trials, and 5) to classify mortality events carefully as to chronology, cause and preventability.